For me it was the benefit of being able to get almost anywhere from LCY by connecting at AMS which made me avoid the stress of Heathrow.

Upon reaching gold elite LHR doesn't seem as bad anymore with the priority lane and the KLM lounge in LHR is one of their best ones. But still all things being equal then LCY is still preferred.

This does seem to be the common reason why British people use KLM, it has an excellent range of UK regional airports served so if you can't get a direct flight from your local airport then there's a good chance you can fly KL via AMS.

Although if KLM's service levels drop too low then more people may stop using them.